Human monocytes are activated by endotoxin and other inflammatory microbial products to produce hormone-like factors such as interleukin 1 (IL 1). We have identified a human monocytic leukemia cell line (THP-1) as a valuable source of human IL 1 for biochemical purification and molecular cloning of the IL 1 gene. IL 1 can be produced from the THP-1 cell line under serum-free condition and has been purified 10,000 fold. Using purified samples of IL 1, we have determined that IL 1 is the mediator which activates chondrocytes to degrade matrix proteoglycan.